Tag official API for module export (#2235)

* functions
 * classes
 * UDLs
 * other declarations

Export everything in namespace 'fmt' from core.h and format.h

include/fmt/ostream.h

@@ -14,9 +14,13 @@
 
 FMT_BEGIN_NAMESPACE
 
+FMT_MODULE_EXPORT_BEGIN
+
 template <typename Char> class basic_printf_parse_context;
 template <typename OutputIt, typename Char> class basic_printf_context;
 
+FMT_MODULE_EXPORT_END
+
 namespace detail {
 
 template <class Char> class formatbuf : public std::basic_streambuf<Char> {
@@ -151,6 +155,7 @@ struct fallback_formatter<T, Char, enable_if_t<is_streamable<T, Char>::value>>
 };
 }  // namespace detail
 
+FMT_MODULE_EXPORT
 template <typename Char>
 void vprint(std::basic_ostream<Char>& os, basic_string_view<Char> format_str,
             basic_format_args<buffer_context<type_identity_t<Char>>> args) {
@@ -168,6 +173,7 @@ void vprint(std::basic_ostream<Char>& os, basic_string_view<Char> format_str,
     fmt::print(cerr, "Don't {}!", "panic");
   \endrst
  */
+FMT_MODULE_EXPORT
 template <typename S, typename... Args,
           typename Char = enable_if_t<detail::is_string<S>::value, char_t<S>>>
 void print(std::basic_ostream<Char>& os, const S& format_str, Args&&... args) {